Differential Slope Calculation 9

[nightfox1925]

I am rusty on this. I remember calculating the % CT mismatch error by comparing primary and secondary currents using an old GE manual for differential relaying.

If I remember it right, is the % slope the sum of this %CT primary-secondary mismatch plus %CT error plus tap changer range (say for example 20% for a +/-10% regulation) plus some % margin?

I appreciate if anyone can provide me a recap...Thanks

 

[nightfox1925]

Thanks Veritas.

I will wait for your comments on the attached calculation trial I made.

I also purchased a book titled "Numerical Differential Protection" By Gerhard Ziegler of Siemens for my reference.

 

[slavag]

Hi Veritas.
This model ( cos Phi) is used in the 316*4/216 ABB series relay.
Nightfox, in lot of cases used max phase current between three for used as restraint current.

Best Regards.
Slava

 

[nightfox1925]

Hi Veritas,

I would like to get your comments on the calculations I have posted on the three winding transformer. Do you have any take on it?

 

[veritas]

Hello nightfox1925

Sorry for the delay in getting back to you. Things have been absolutely hectic. Anyway, I've had a look at your calcs and at first all looked okay. But then a slope setting of 80% where the max voltage boost is 0.9 p.u. just did not seem right for a 3 winding trfr. Actually if you did your calcs with the tap at nominal position, Id = abs(1 - (1+1)) = 1 and Itot (or Irest as you would call it) = 1. This gives a slope of 100% - no something ain't right. After all if it was a 4 winding trfr using this methodology your slope would come out to be 200%!!!

The problem is that you are doing your calcs at the base of each winding and not to a common base. I would thus suggest you do your calcs in per unit.

For example, considering nominal tap again,

a. IFLA (30MVA winding) = 240.56A as before, or 30/30 = 1p.u.

Similarly

b. IFLA (13MVA winding) = 13/30 = 0.433p.u.

c. IFLA (17MVA winding) = 17/30 = 0.567p.u.


Now Id = abs(1 - (0.433 + 0.567)) = 0 as expected.

You can now use the above methodology to calculate the Id and Itot for when the trfr is at maximum boost. Only now you need to use turns ratio in per unit and not actual turns ratio.

Hope this helps.

Veritas

 

[nightfox1925]

Thanks Veritas. I'll try to re-calculate using per-unit. I'll post the results later. Again, I appreciate your effort gettibng back to me.

 

[veritas]

You're welcome nightfox. Good luck.

Kind regards.

Veritas